varsup.c

/*------------------------------------------------------------------------- * * varsup.c * postgres OID & XID variables support routines * * Copyright (c) 2000-2009, PostgreSQL Global Development Group * * IDENTIFICATION * $PostgreSQL: pgsql/src/backend/access/transam/varsup.c,v 1.84 2009/04/23 00:23:45 tgl Exp $ * *------------------------------------------------------------------------- */#include "postgres.h"#include "access/clog.h"#include "access/subtrans.h"#include "access/transam.h"#include "miscadmin.h"#include "postmaster/autovacuum.h"#include "storage/pmsignal.h"#include "storage/proc.h"#include "utils/builtins.h"/* Number of OIDs to prefetch (preallocate) per XLOG write */#define VAR_OID_PREFETCH 8192/* pointer to "variable cache" in shared memory (set up by shmem.c) */
VariableCache ShmemVariableCache = NULL;
/* * Allocate the next XID for my new transaction or subtransaction. * * The new XID is also stored into MyProc before returning. */
TransactionId
GetNewTransactionId(bool isSubXact)
{
TransactionId xid;
/* * During bootstrap initialization, we return the special bootstrap * transaction id. */if (IsBootstrapProcessingMode())
{
Assert(!isSubXact);
MyProc->xid = BootstrapTransactionId;
return BootstrapTransactionId;
}
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
xid = ShmemVariableCache->nextXid;
/*---------- * Check to see if it's safe to assign another XID. This protects against * catastrophic data loss due to XID wraparound. The basic rules are: * * If we're past xidVacLimit, start trying to force autovacuum cycles. * If we're past xidWarnLimit, start issuing warnings. * If we're past xidStopLimit, refuse to execute transactions, unless * we are running in a standalone backend (which gives an escape hatch * to the DBA who somehow got past the earlier defenses). * * Test is coded to fall out as fast as possible during normal operation, * ie, when the vac limit is set and we haven't violated it. *---------- */if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit) &&
TransactionIdIsValid(ShmemVariableCache->xidVacLimit))
{
/* * To avoid swamping the postmaster with signals, we issue the autovac * request only once per 64K transaction starts. This still gives * plenty of chances before we get into real trouble. */if (IsUnderPostmaster && (xid % 65536) == 0)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
if (IsUnderPostmaster &&
TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidStopLimit))
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands to avoid wraparound data loss in database \"%s\"",
NameStr(ShmemVariableCache->limit_datname)),
errhint("Stop the postmaster and use a standalone backend to vacuum database \"%s\".\n""You might also need to commit or roll back old prepared transactions.",
NameStr(ShmemVariableCache->limit_datname))));
elseif (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidWarnLimit))
ereport(WARNING,
(errmsg("database \"%s\" must be vacuumed within %u transactions",
NameStr(ShmemVariableCache->limit_datname),
ShmemVariableCache->xidWrapLimit - xid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in \"%s\".\n""You might also need to commit or roll back old prepared transactions.",
NameStr(ShmemVariableCache->limit_datname))));
}
/* * If we are allocating the first XID of a new page of the commit log, * zero out that commit-log page before returning. We must do this while * holding XidGenLock, else another xact could acquire and commit a later * XID before we zero the page. Fortunately, a page of the commit log * holds 32K or more transactions, so we don't have to do this very often. * * Extend pg_subtrans too. */
ExtendCLOG(xid);
ExtendSUBTRANS(xid);
/* * Now advance the nextXid counter. This must not happen until after we * have successfully completed ExtendCLOG() --- if that routine fails, we * want the next incoming transaction to try it again. We cannot assign * more XIDs until there is CLOG space for them. */
TransactionIdAdvance(ShmemVariableCache->nextXid);
/* * We must store the new XID into the shared ProcArray before releasing * XidGenLock. This ensures that every active XID older than * latestCompletedXid is present in the ProcArray, which is essential for * correct OldestXmin tracking; see src/backend/access/transam/README. * * XXX by storing xid into MyProc without acquiring ProcArrayLock, we are * relying on fetch/store of an xid to be atomic, else other backends * might see a partially-set xid here. But holding both locks at once * would be a nasty concurrency hit. So for now, assume atomicity. * * Note that readers of PGPROC xid fields should be careful to fetch the * value only once, rather than assume they can read a value multiple * times and get the same answer each time. * * The same comments apply to the subxact xid count and overflow fields. * * A solution to the atomic-store problem would be to give each PGPROC its * own spinlock used only for fetching/storing that PGPROC's xid and * related fields. * * If there's no room to fit a subtransaction XID into PGPROC, set the * cache-overflowed flag instead. This forces readers to look in * pg_subtrans to map subtransaction XIDs up to top-level XIDs. There is a * race-condition window, in that the new XID will not appear as running * until its parent link has been placed into pg_subtrans. However, that * will happen before anyone could possibly have a reason to inquire about * the status of the XID, so it seems OK. (Snapshots taken during this * window *will* include the parent XID, so they will deliver the correct * answer later on when someone does have a reason to inquire.) */
{
/* * Use volatile pointer to prevent code rearrangement; other backends * could be examining my subxids info concurrently, and we don't want * them to see an invalid intermediate state, such as incrementing * nxids before filling the array entry. Note we are assuming that * TransactionId and int fetch/store are atomic. */volatile PGPROC *myproc = MyProc;
if (!isSubXact)
myproc->xid = xid;
else
{
int nxids = myproc->subxids.nxids;
if (nxids < PGPROC_MAX_CACHED_SUBXIDS)
{
myproc->subxids.xids[nxids] = xid;
myproc->subxids.nxids = nxids + 1;
}
else
myproc->subxids.overflowed = true;
}
}
LWLockRelease(XidGenLock);
return xid;
}
/* * Read nextXid but don't allocate it. */
TransactionId
ReadNewTransactionId(void)
{
TransactionId xid;
LWLockAcquire(XidGenLock, LW_SHARED);
xid = ShmemVariableCache->nextXid;
LWLockRelease(XidGenLock);
return xid;
}
/* * Determine the last safe XID to allocate given the currently oldest * datfrozenxid (ie, the oldest XID that might exist in any database * of our cluster). */void
SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
Name oldest_datname)
{
TransactionId xidVacLimit;
TransactionId xidWarnLimit;
TransactionId xidStopLimit;
TransactionId xidWrapLimit;
TransactionId curXid;
Assert(TransactionIdIsNormal(oldest_datfrozenxid));
/* * The place where we actually get into deep trouble is halfway around * from the oldest potentially-existing XID. (This calculation is * probably off by one or two counts, because the special XIDs reduce the * size of the loop a little bit. But we throw in plenty of slop below, * so it doesn't matter.) */
xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
if (xidWrapLimit < FirstNormalTransactionId)
xidWrapLimit += FirstNormalTransactionId;
/* * We'll refuse to continue assigning XIDs in interactive mode once we get * within 1M transactions of data loss. This leaves lots of room for the * DBA to fool around fixing things in a standalone backend, while not * being significant compared to total XID space. (Note that since * vacuuming requires one transaction per table cleaned, we had better be * sure there's lots of XIDs left...) */
xidStopLimit = xidWrapLimit - 1000000;
if (xidStopLimit < FirstNormalTransactionId)
xidStopLimit -= FirstNormalTransactionId;
/* * We'll start complaining loudly when we get within 10M transactions of * the stop point. This is kind of arbitrary, but if you let your gas * gauge get down to 1% of full, would you be looking for the next gas * station? We need to be fairly liberal about this number because there * are lots of scenarios where most transactions are done by automatic * clients that won't pay attention to warnings. (No, we're not gonna make * this configurable. If you know enough to configure it, you know enough * to not get in this kind of trouble in the first place.) */
xidWarnLimit = xidStopLimit - 10000000;
if (xidWarnLimit < FirstNormalTransactionId)
xidWarnLimit -= FirstNormalTransactionId;
/* * We'll start trying to force autovacuums when oldest_datfrozenxid gets * to be more than autovacuum_freeze_max_age transactions old. * * Note: guc.c ensures that autovacuum_freeze_max_age is in a sane range, * so that xidVacLimit will be well before xidWarnLimit. * * Note: autovacuum_freeze_max_age is a PGC_POSTMASTER parameter so that * we don't have to worry about dealing with on-the-fly changes in its * value. It doesn't look practical to update shared state from a GUC * assign hook (too many processes would try to execute the hook, * resulting in race conditions as well as crashes of those not connected * to shared memory). Perhaps this can be improved someday. */
xidVacLimit = oldest_datfrozenxid + autovacuum_freeze_max_age;
if (xidVacLimit < FirstNormalTransactionId)
xidVacLimit += FirstNormalTransactionId;
/* Grab lock for just long enough to set the new limit values */
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
ShmemVariableCache->oldestXid = oldest_datfrozenxid;
ShmemVariableCache->xidVacLimit = xidVacLimit;
ShmemVariableCache->xidWarnLimit = xidWarnLimit;
ShmemVariableCache->xidStopLimit = xidStopLimit;
ShmemVariableCache->xidWrapLimit = xidWrapLimit;
namecpy(&ShmemVariableCache->limit_datname, oldest_datname);
curXid = ShmemVariableCache->nextXid;
LWLockRelease(XidGenLock);
/* Log the info */
ereport(DEBUG1,
(errmsg("transaction ID wrap limit is %u, limited by database \"%s\"",
xidWrapLimit, NameStr(*oldest_datname))));
/* * If past the autovacuum force point, immediately signal an autovac * request. The reason for this is that autovac only processes one * database per invocation. Once it's finished cleaning up the oldest * database, it'll call here, and we'll signal the postmaster to start * another iteration immediately if there are still any old databases. */if (TransactionIdFollowsOrEquals(curXid, xidVacLimit) &&
IsUnderPostmaster)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
/* Give an immediate warning if past the wrap warn point */if (TransactionIdFollowsOrEquals(curXid, xidWarnLimit))
ereport(WARNING,
(errmsg("database \"%s\" must be vacuumed within %u transactions",
NameStr(*oldest_datname),
xidWrapLimit - curXid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in \"%s\".\n""You might also need to commit or roll back old prepared transactions.",
NameStr(*oldest_datname))));
}
/* * GetNewObjectId -- allocate a new OID * * OIDs are generated by a cluster-wide counter. Since they are only 32 bits * wide, counter wraparound will occur eventually, and therefore it is unwise * to assume they are unique unless precautions are taken to make them so. * Hence, this routine should generally not be used directly. The only * direct callers should be GetNewOid() and GetNewRelFileNode() in * catalog/catalog.c. */
Oid
GetNewObjectId(void)
{
Oid result;
LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
/* * Check for wraparound of the OID counter. We *must* not return 0 * (InvalidOid); and as long as we have to check that, it seems a good * idea to skip over everything below FirstNormalObjectId too. (This * basically just avoids lots of collisions with bootstrap-assigned OIDs * right after a wrap occurs, so as to avoid a possibly large number of * iterations in GetNewOid.) Note we are relying on unsigned comparison. * * During initdb, we start the OID generator at FirstBootstrapObjectId, so * we only enforce wrapping to that point when in bootstrap or standalone * mode. The first time through this routine after normal postmaster * start, the counter will be forced up to FirstNormalObjectId. This * mechanism leaves the OIDs between FirstBootstrapObjectId and * FirstNormalObjectId available for automatic assignment during initdb, * while ensuring they will never conflict with user-assigned OIDs. */if (ShmemVariableCache->nextOid < ((Oid) FirstNormalObjectId))
{
if (IsPostmasterEnvironment)
{
/* wraparound in normal environment */
ShmemVariableCache->nextOid = FirstNormalObjectId;
ShmemVariableCache->oidCount = 0;
}
else
{
/* we may be bootstrapping, so don't enforce the full range */if (ShmemVariableCache->nextOid < ((Oid) FirstBootstrapObjectId))
{
/* wraparound in standalone environment? */
ShmemVariableCache->nextOid = FirstBootstrapObjectId;
ShmemVariableCache->oidCount = 0;
}
}
}
/* If we run out of logged for use oids then we must log more */if (ShmemVariableCache->oidCount == 0)
{
XLogPutNextOid(ShmemVariableCache->nextOid + VAR_OID_PREFETCH);
ShmemVariableCache->oidCount = VAR_OID_PREFETCH;
}
result = ShmemVariableCache->nextOid;
(ShmemVariableCache->nextOid)++;
(ShmemVariableCache->oidCount)--;
LWLockRelease(OidGenLock);
return result;
}